Tap changers are widely used today to regulate and control the output of large or medium sized inductive power devices, such as transformers and reactors. These inductive power devices transform electric power from a first power system to a second power system. As the load of the second system increases, the output voltage of the inductive power device drops. In order to compensate for this output voltage drop, i.e. maintain a relatively constant output voltage level in the second power system, or adjust the output voltage in accordance with power board customers requirements, the tap changers are used.
The tap changer controls and regulates the voltage transformation for transformers and inductance for reactors. In a transformer, the voltage of the second system depends on a turn ratio, i.e. the ratio between the number of turns between the secondary and primary winding of the transformer. The tap changer, most often being connected to the primary winding, affects this turn ratio by performing a switching between different fixed contacts or taps of the winding. This switching is typically performed in small steps resulting in a voltage change of about 0.5–2.5%, usually around 1.5%, of the rated voltage per step.
The tap changer becomes heated by its operations but also by the load and is therefore immersed in a cooling fluid, most often oil. The cooling fluid acts at the same time as an insulating medium. But since arcing normally occur during switching operations the oil may deteriorate and deterioration products are formed in the fluid. In order not to contaminate the inductive power device, the tap changer is preferably separated from the fluid of the inductive power device and is placed in fluid filled tank, in turn being located either in or on the tank of the inductive power device.
Since the tap changer and the fluid become heated during operation, a monitoring of the tap changer is often employed to maintain a reliable and efficient operation. In such a monitoring, several parameters may be measured and observed to get a picture of the overall condition of the tap changer. In particular the temperature of the cooling and insulating fluid needs to be kept below a certain safety limit. Breakdowns, possibly leading to the destruction of the entire or part of the tap changer or a shortening of its life may happen if the fluid temperature exceeds certain limits. In some situations also the inductive power device may be affected, damaging its equipment of shortening its life.
In the patent JP 10041162 an abnormality diagnostic device for a tap changer is disclosed. The device has a torque detector to detect a torque of a drive shaft of the tap changer. A temperature detector detects the temperature of the tap changer. Based on this detected temperature, a correction operation is carried out on a standard torque pattern. The corrected standard torque pattern is then compared with the detected torque pattern of the drive shaft in order to diagnose any abnormality.
U.S. Pat. No. 6,052,060 discloses an oil immersed electric power switch-gear, such as a tap changer, with an oil circulating system filtering formed carbon particles from the oil. The circulating system is provided with a temperature probe for measuring the temperature of the oil leaving a tank containing the tap changer. The measured temperature is compared with setpoint values to detect a possible too high oil temperature. In order to prevent or minimise equipment damage due to the high oil temperature alarms are issued and/or electrical loads are disconnected.
Most prior art monitor and diagnosing systems, such as the one described in U.S. Pat. No. 6,052,060 above, concern protection against acute damage to the tap changer and its associated equipment. The systems most often base the condition diagnosing of the tap changer on a direct temperature measurement of the cooling and insulating fluid and perform actions if the temperature exceeds certain safety thresholds. Thus, any actions are only taken if the fluid temperature is high enough and the situation therefore is or soon may become critical. Such emergency actions may include load reduction and in extreme conditions shutting down the tap changer and inductive power device. However, a malfunctioning tap changer may not necessarily lead to a dangerous heating at all instances. A malfunctioning tap changer may e.g. operate at a low load and will then, despite its defects, operate at a permitted temperature. No indication of the defect is detected until the load and power is increased. At such an occasion, it is often very inconvenient to take the tap changer out of operation for repair, since it typically coincides with an increased performance demand.
It would therefore be advantageously if one, as a complement to the conventional overtemperature protection devices, could achieve an apparatus for providing information about the actual operation condition, related to heat issues, of the tap changer at any instant, i.e. a condition diagnosing apparatus.